Scientists from the University of California, Irvine, describe in the July 8 issue of PNAS a molecule for volumetric optical storage that exhibits widely separate write, read and erase bands. Rewritable 3-D memory promises to enable the development of terabyte-capacity removable storage media for applications such as telemedicine, high-definition video and defense.The new material combines a photochromic fulgimide and a fluorescent oxazine dye, which are chemically bonded to form a single molecule with unique optical properties. When exposed to 530-nm light, the fulgimide switches from a closed polar form to an open nonpolar one, which reverts to the polar form when excited by 400-nm light. The dye has a peak absorption at approximately 650 nm and fluoresces at 700 nm, but when it is part of the composite molecule, it will do so only when the fulgimide component is in its nonpolar form. Because the various absorption wavelengths are so different, writing, reading or erasing the material does not cause the molecule to take an unwanted form.To demonstrate the feasibility of the composite molecules for optical storage, the researchers fixed the material in a layer of polymethylmethacrylate and used a two-photon absorption process to form images with a series of open and closed molecules. Repeated erasing and rewriting of the data had no effect on the intensity of the fluorescent response.The researchers note that volumetric storage media have been limited to ROM and WORM configurations. They say that other materials that have been examined for rewritable memory suffer from destructible read-out problems.